Feat: support specifying more Gateways to indexer config

[calinah]

As work towards launching more gateways in the near future is ongoing we will need to either:

• document how indexers are expected to accept receipts from multiple gateway - in the event an indexer is expected to run multiple indexer stacks for each gateway they accept receipts from

• update indexer-agent cli option --collect-receipts-endpoint or --gateway-endpoint to support multiple endpoints

• update indexer-service cli option --client-signer-address to support passing multiple gateway addresses